Significance Daylight vision results from the absorption of light by photosensitive pigments located in retinal cones. The spectral properties of these pigments limit sensitivity and determine the nature of color vision. Objectives Platyrrhine monkeys often show striking polymorphic variations I the complement of photopigments found in their cone photoceptors. These polymorphisms are directly linked to significant variations in color vision. The purpose of this research is to determine if Titi monkeys (Callicebus moloch) have cone photopigment polymorphisms and, if so, to understand the mechanisms underlying this variation. Results A nonevasive electrophysiological procedure, electroretinogram flicker photometry, was used to measure the photopigments in Titi monkeys. Like other platyrrhine monkeys, this species has a sex-linked photopigment polymorphism. All male monkeys have only two types of cone photopigments. This indicates that they will have dichromatic color vision. Among the males, five different phenotypes were detected. Most females have three different types of cone photopigments and, thus, will have trichromatic color vision. There is a significant variation among the females in the nature if the three pigments they possess. Examination of photopigment pedigrees provides evidence that these polymorphisms arise from X-linked opsin genes. The range of variation in this species is unique and thus provides fascinating evidence about the evolution of primate color vision. Future Directions Examination of the structure of the opsin genes of Titi monkeys will permit a better understanding of the relationships between genes, photopigments, and color vision in primates. KEY WORDS photopigment, opsin genes, color vision, evolution FUNDING National Science Foundation IBN-9318770